st: Question about "predict" and "mean" when i ~= (not equal) j?

Hi, all,
Theoretically, my question is easy; but I do not know how to program it
in STATA.
Y is my dependent variable, X1, X2, X3 are my independent variable,
i/j=1, 2, ..., n are my observations, m=1, 2, ...,20 is city, my
equation is Y==X1 X2 X3. What I want to calculate is GAP(i~=j,
m)=Predicted Y(j) for i-mean(j, m) (j is all the other observations). I
know how to do for the equation GAP(i,m)=Predicted Y(i)-mean(i,m) , in
this case, what I do is
Reg Y X1 X2 X3
Predict PY, xb
Gen gap=PY-Y
collapse gap, by(m)
But I do not know how to predict and calculate the value which does not
include the current observation (the predicted value and the mean value
which excludes the current value)?
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